Conventionally, in a power conversion device used for electric vehicles, AC power collected from overhead wires via pantographs is converted by a converter into DC power. Alternatively, the DC power is directly collected from pantographs. The DC power is inputted to an inverter through a capacitor, which is used for energy storage and suppresses fluctuation in the power-supply voltage. The DC power inputted to the inverter is converted to AC power having a variable voltage and a variable frequency, and the converted AC power is supplied to a synchronous machine of a permanent-magnet type.
Here, since the synchronous machine mounts therein a permanent magnet, the synchronous machine constantly induces electromotive force even during its rotation. The induced voltage becomes large in proportion to the speed, and when an electric motor vehicle runs at a high speed, the induced voltage exceeding the power-supply voltage will be generated. Thus, during a power running operation, the driving torque of the synchronous machine is controlled by so-called weak field control, i.e., by controlling the induced voltage as well as the current of the synchronous machine by means of the inverter, and during a regenerative operation, the braking torque of the synchronous machine is controlled.
Meanwhile, in the case of controlling the electric motor vehicles using a synchronous machine of a permanent-magnet type, an inverter is usually stopped during coasting. However, when the inverter is stopped, the synchronous machine works as a generator, and the regenerated energy flows to the power supply side through a diode in the inverter, and as a result, the electric motor vehicle performs a braking operation instead of a coasting operation.
In order to prevent such a phenomenon, in the conventional technology, a load contactor is arranged between a synchronous machine and an inverter, and during the coasting operation, the load contactor arranged between the synchronous machine and the inverter is opened (OFF state) and the power supply line is blocked so as to prevent the regenerated energy of the synchronous machine from flowing to the inverter. In addition, at the time of re-activation, i.e., shifting from the coasting operation to the power running operation, in order to prevent unnecessary flow of regenerated energy into the inverter and the consequent damages of switching elements of the inverter, the converter is controlled before the load contactor is powered ON so that the voltage of a direct current side circuit of the inverter is boosted to be equal to or higher than the peak value of the induced voltage between terminals of the synchronous machine, and then the load contactor is powered ON (e.g., see Patent document 1 listed below).